This invention relates to a DC voltage control circuit, and more particularly a DC voltage control circuit utilized in a demodulation apparatus as a circuit for discriminating a demodulated signal produced by phase detecting a quadrature amplitude modulated wave, and for compensating for the DC drift or the like of the demodulated signal.
In recent years, a multi-level quadrature amplitude modulation (QAM) system for carrying a digital signal with a microwave carrier wave has been developed as a high efficiency modulation system capable of efficiently utilizing a frequency band. In order to realize such a high efficiency modulation system, it is necessary to avoid, as far as possible, undesirable phenomenon occurring at various parts of the circuit, a typical one of such phenomena being a DC drift contained in the input to the discriminating circuit of the demodulation apparatus.
In a DC voltage control circuit for compensating for the DC drift component, a DC superposing circuit such as a subtractor superposes a DC voltage upon the demodulated multi-level signal. A discriminating circuit such as an A/D (analog-to-digital) converter discriminates the output of the superposing circuit with respect to a plurality of discriminating levels and provides a plurality of digital signals. The control signal generator filters one of the digital signals and produces the DC voltage. This circuit operates satisfactory when used continuously. But, as will be described later, when the input signal is once interrupted and then recovered, there is a fear that an abnormal pull-in occurs depending upon the initial state. Therefore, the circuit provides erroneous DC voltage, thereby failing to resume a normal operation.